Unused var
Allow cwd to be configured for build

By default, all paths in the build config (e.g. externs) are assumed to be relative to the ol3 repo root.  To make it so relative paths are resolved from a different directory, use the cwd option.

In addition, this change makes it so the `build.js` task can be run from another directory.
Merge pull request #2275 from tschaub/relative

Allow build task to be run from anywhere and provide option for resolving relative paths.